1. Field of the Invention
This invention relates to thin film materials having a perovskite structure. More particularly, a method of using an electrical pulse to modify or switch the characteristics of materials such as colossal magnetoresistive and high temperature superconducting materials is provided and applications of the method are provided.
2. Description of Related Art
The materials having a perovskite structure, among them colossal magnetoresistance (CMR) materials and high temperature superconductivity (HTSC) materials, are important in many fields. The common and the most substantial characteristic of the CMR and HTSC materials is that their electrical resistance can be changed significantly by external influence, such as temperature, magnetic field or electric field. The CMR materials have wide applications. They can provide a cheap and practical means of sensing magnetic fields ("Colossal Magnetoresistance not just a Load of Bolometers," N. Mathur, Nature, vol. 390, pp. 229-231, 1997), and lead to dramatic improvements in the data density and reading speed of magnetic recording systems ("Thousandfold Change in Resistivity in Magnetoresistive La--Ca--Mn--O Films," S. Jin, et al, Science, vol. 264, pp. 413-415, 1994). They can also become a new material for thermal or infrared detectors, and a new material for photo and X-ray detection ("Photoinduced Insulator-to-Metal Transition in a Perovskite Manganite," K. Miyano, et al, Physical Review Letters, vol. 78, pp. 4257-4260, 1997; "An X-ray-induced Insulator-metal Transition in a Magnetoresistive Manganite," V. Klyukhin, et al, Nature, vol. 386, pp. 313-315, 1997). Moreover, a static electric field can trigger the collapse of the insulating charge-ordered state of CMR materials to a metallic ferromagnetic state, and so provide a route for fabricating micrometer- or nanometer-scale electromagnets ("Current Switching of Resistive States in Magnetoresistive Manganites," A. Asamitsu, et al, Nature, vol. 388, pp. 50-52, 1997). The main use of the recently discovered HTSC materials is, of course, as the superconductors, and their conductivity can be affected by applied electric current or magnetic field ("Transient Behavior and Memory Effect of a PbZr.sub.x Ti.sub.1-x O.sub.3 /YBa.sub.2 Cu.sub.3 O.sub.7-x Three-terminal Device," H. Lin, et al, Appl. Physics Letters, vol. 65, pp. 953, 1994). However, they also have some other uses, for example as the conductive layer in the epitaxial multilayer structures used at room temperature ("Heterostructures of Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 and YBa.sub.2 Cu.sub.3 O.sub.7-.delta. on MgO Substrate Prepared by Pulsed Laser Ablation, N. J. Wu, et al, Jpn. J. Appl. Phys., vol. 32, pp. 5019-5023, 1993).
Although it has been known that temperature, magnetic field, or static electric field can change the properties of the CMR and HTSC materials, it has been observed that these stimuli do not switch the states or permanently modify the properties of these materials. Therefore, when the stimulus vanishes, the changed material's state or property will return back to its original value. For example, the resistance of CMR materials changes with large applied magnetic fields. When the magnetic field increases, the resistance of a CMR material decreases, and when the magnetic field decreases and goes back to zero, the resistance of the CMR material will increase and return to its original value. Only at very low temperatures does a relatively large resistive lag occur in these materials, which is somewhat like the magnetic lag for ferromagnets.
Another limitation of previously observed changes in the properties of CMR materials as a result of a stimulus is that they significantly respond to changes in magnetic field only under large magnetic fields (several Tesla), or changes in static electric field only at very low temperatures. Therefore, it is not convenient in many cases to use magnetic or static electric fields to change the properties of the CMR materials. As many of the current applications of CMR and HTSC materials are based on using thin films, there is great interest in modifying thin film properties of these materials.
What is needed is a method which can be used to repeatedly switch the state or modify the properties of CMR, HTSC and films of other perovskite-like materials such that a modified property will continue after the stimulus to switch is removed. The method should work conveniently, repeatedly and at room temperature.